Drape strings over a DTM

You can use this function to drape strings over a DTM. At the same time, you can use this function to perform basic maths operations between a string value and a surface value at the same X,Y location. You can also choose to create new points wherever a string intersects a triangle edge. This allows you to create a string that follows the relief in the DTM.

Examples of reasons you might want to drape strings over a DTM include:

to transfer the elevations of a DTM into the Z field of a string file

For example, after digitising some surface features, this function can be used to assign the correct elevations to the digitised features.

to obtain the difference in elevation between two surfaces for the subsequent generation of isopach (thickness) contours
to obtain stripping ratios of overburden thickness to seam thicknesses
to overlay a grid string onto a DTM to generate a three dimensional mesh

To run this function: Choose Surfaces > DTM file functions > Drape strings over a DTM, or...

In the Function Chooser, type STRINGS OVER DTM, and press ENTER.

You can use a boundary string to set a limit on which points in the strings are draped onto the DTM.

Fields on the Overlay strings on a DTM form

Field	Description
Define the DTM
Location	The name and location of the DTM that you want to drape strings over.
[Refresh]	Loads the Object ID and Trisolation ID drop-down lists with the objects and trisolations of the DTM. In Surpac 6.6.1 and earlier versions, when you selected a DTM in the Location field, Surpac would load the DTM, validate it, and then load the Object ID and Trisolation ID drop-down lists. You could not interact with the form until the process was complete, which could be a long time for some large DTMs. Now, Surpac does not load the drop-down lists until you click the Refresh button, and validation does not occur until you click Apply in the form. Any validation errors are reported in the Message window.
ID number	The ID number of the DTM that you want to drape strings over.
Object ID	The object number of the DTM that you want to drape strings over.
Trisolation ID	The trisolation number of the DTM that you want to drape strings over.
Use descriptions	Selected: The description fields in the strings are retained. Cleared: The description fields in the strings are not retained.
Handle descriptions as	Default: The description field is treated as either a mathematical formula or a text description depending on the contents of the field when interpolating points. Text: The description field is treated as a text description when interpolating points. Math: The description field is treated as a mathematical formula when interpolating points.
Define the input strings
Location	The name and location of the string file that contains the strings that you want to drape over the DTM.
Id number	The ID number of the string file that contains the strings that you want to drape over the DTM.
String range	The range of strings within the string file that you want to drape over the DTM. If you leave this field blank, all strings are selected.
Define the output strings
Interpolate new points	Selected: New points are created, by linear interpolation, where the string intersects the edge of a triangle. Cleared: No new points are created.
Location	The name and location for the new string file containing the points of the strings after they have been draped onto the DTM.
ID number	The ID number of the new string file.
Define the boundary string
Use a boundary string	Selected: A boundary string is used to limit the points in the string file that are draped onto the DTM. Points that are inside the boundary string are draped onto the DTM, and points outside the string are not changed. Cleared: A boundary string is not used. The boundary string can consist of any number of nested segments. The direction of these segments (clockwise or anticlockwise) is not important. Imagine that you are viewing a surface along the normal to the plane used during surface creation (that is, looking directly at the plane), the rules for clipping are: For a single segment string, "inside" the segment refers to triangles physically inside the segment. For a string with a single level of nested segments, "inside" refers to triangles physically inside the outermost segment but outside any inner segments. Multiple levels of nesting are handled by alternating "inside" and "outside" for each level of nesting. For example, if the black lines are segments of a string, the red areas are considered "inside" the string and the white areas are "outside":
Location	The name and location of the string file that contains the string you want to use as the boundary.
ID number	The ID number of the string file that contains the string you want to use as the boundary.
String number	The number of the string that you want to use as the boundary.

After you apply the Overlay strings on a DTM form, the Define required operations form appears.

Fields on the Define required operations form

Field	Description
Str field	The field from the string file to which the operation is to be performed. If you did not select Use descriptions the only option is Z. Note: This field must contain numeric data.
Operator	The operation to be applied to the string field defined above must be entered. Valid operations are: = Make string value equal DTM value. + Add string value to DTM value. - Subtract DTM value from string value. * Multiply string value by DTM value. / Divide string value by DTM value.
DTM field	The field in the DTM that is used to modify the field in the string according to the operator selected.

Field

Description

Str field

The field from the string file to which the operation is to be performed. If you did not select Use descriptions the only option is Z.

Note: This field must contain numeric data.

Operator

The operation to be applied to the string field defined above must be entered. Valid operations are:

= Make string value equal DTM value.
+ Add string value to DTM value.
- Subtract DTM value from string value.
* Multiply string value by DTM value.
/ Divide string value by DTM value.

DTM field

The field in the DTM that is used to modify the field in the string according to the operator selected.

Output

A new string file is created in which the Z value of each point on the input strings is modified so that it is equal to the Z value of the DTM at the same X,Y location. If you selected Interpolate new points, new points are created in the string where the string crosses the edge of a triangle.

The fields in the strings that mathematical operations were applied to are modified and saved in the string file.

When draping strings on a DTM created on a non-horizontal plane, the process is:

The DTM and input string data are transformed so that the plane is horizontal.
The overlaying process proceeds as for a horizontal plane surface.
The results are back-transformed to sit on the original DTM location.